CS 46B - Lecture 6

Pre-class reading

• Sections 11.4 - 11.6

Checked Exceptions

• new Scanner(file) can throw FileNotFoundException
• Checked exception—can't ignore
• IO exceptions are checked since they are beyond the control of the programmer
• Use throws to alert the caller

  public static void process(String filename) throws FileNotFoundException
  
  public static void main(String[] args) throws FileNotFoundException

Lecture 6 Clicker Question 1

When you call a[i] on an array, your code might throw an ArrayIndexOutOfBoundsException. Why don't you have to tag methods with throws ArrayIndexOutOfBoundsException?

• Because ArrayIndexOutOfBoundsException is a checked exception
• Because the ArrayIndexOutOfBoundsException is checked at compile-time
• Because you could have checked whether i is valid
• Because the code could have also thrown a NullPointerException (if a is null)

Catching Exceptions

try
{
   String filename = . . .;
   FileReader reader = new FileReader(filename);
   Scanner in = new Scanner(reader);
   String input = in.next();
   int value = Integer.parseInt(input);
   . . .
}
catch (IOException exception)
{
   exception.printStackTrace();
}
catch (NumberFormatException exception)
{
   System.out.println("Input was not a number");
}

Exceptions and Inheritance

• Can use superclass in throws, catch

Lecture 6 Clicker Question 2

try 
{
   process(filename)
}
catch (FileNotFoundException ex) { println("File not found"); }
catch (IOException ex) { println("I/O error"); }
catch (RuntimeException ex) { println("Something went wrong"); }}

Suppose process throws a EOFException. What gets printed?

The finally Clause

• Exception terminates current method
• Danger: Can skip over essential code
• Example:
  

  reader = new FileReader(filename);
  Scanner in = new Scanner(reader);
  readData(in);
  reader.close(); // May never get here

• Must execute reader.close() even if exception happens
• Use finally clause for code that must be executed no matter what

The finally Clause

Scanner in = new Scanner(file);
try
{
   readData(in);
}
finally
{
   in.close(); 
}

• Executed when try block is exited in any of three ways:
  1. After last statement of try block
  2. After last statement of catch clause, if this try block caught an exception
  3. When an exception was thrown in try block and not caught
• Java 7 Shortcut

  try (Scanner in = new Scanner(file))
  {   
     readData(in);
  } // in.close called automatically—Scanner is AutoCloseable

Lecture 6 Clicker Question 3

public void read(String filename) 
{
   Scanner in = null; 
   try
   {
      in = new Scanner(new File(filename));
      process(in);
   }
   catch (IOException ex) { println("I/O Error"); }
   finally { in.close(); }
}

Which of the following is true if the file does not exist?

1. The message "I/O error" is printed
2. A FileNotFoundException is thrown to the caller of read
3. A NullPointerException is thrown to the caller of read
4. The read method returns without printing anything and without an exception